1. Field of the Invention
The invention relates to a sheet feeding apparatus for supplying a sheet such as recording paper to an image forming apparatus such as a printer, a copying machine or a compound machine.
2. Description of Related Art
Among sheet feeding apparatuses, there is one which feeds sheets stacked on an stacking plate successively from the uppermost one of them to the image forming portion of an image forming apparatus by a sheet feeding roller, and such a sheet feeding apparatus is designed such that a rockably provided stacking plate is upwardly biased by a spring so that the uppermost surface of stacked sheets may be brought into pressure contact with a sheet feeding roller. There is also a sheet feeding apparatus designed such that as described in Japanese Patent Application Laid-open No. H04-350033, a sheet feeding cam is fixed coaxially with a sheet feeding roller, and in the course wherein the sheet feeding roller is rotated to thereby feed out a sheet, the sheet feeding cam depresses an stacking plate and lower the stacking plate to a predetermined position against the resilient force of a coil spring.
In a construction wherein as described above, the stacking plate is depressed by the sheet feeding cam, the stacking plate can be lowered to the predetermined position during waiting and therefore, the setting or interchange of the sheets is easy, and the stacking plate is lowered in the course wherein the sheet is fed out, whereby the separability of the sheet by separating means such as a separating pad can be improved.
However, in the sheet feeding apparatus of such a construction in which the stacking plate is automatically lowered to the predetermined position against the resilient force of the coil spring by the sheet feeding cam, the position of the stacking plate biased by the coil spring during sheet feeding differs in accordance with the stack amount (stack height) of the stacked sheets and therefore, the time from the start of the rotation of the sheet feeding roller until the sheet feeding cam abuts against the stacking plate to thereby depress the stacking plate differs depending on the stack amount of the sheets on the stacking plate. Therefore, there has been the problem that the timing at which the sheets stacked on the stacking plate are spaced apart from the sheet feeding roller becomes irregular and the separating performance of the separating means such as the separating pad is unstable.
Also, there has been the problem that the distance by which the uppermost surface of the sheets and the sheet feeding roller in the waiting state are spaced differs between a case where the stack amount of the sheets is great and a case where the stack amount of the sheets is small, and the time until during sheet feeding, the stacking plate is brought from its lowered position in the waiting state to a position in which the upper surface of the sheets comes into pressure contact with the sheet feeding roller differs and therefore, the feed timing for feeding out the sheet deviates and the sheet feeding interval (the interval between the sheets during continuous feeding) does not become constant. Further, there has also been the problem that when the stack amount of the sheets stacked on the stacking plate is small, the amount of movement of the sheets during the movement of the stacking plate between its lowered position and its elevated position becomes great, thus aggravating the aligning property of the stacked sheets. If the aligning property of the sheets is bad, there will arise the problem that when a sheet has been fed out, skew feeding is caused and a faulty image or jam or the like is liable to occur.
So, in order to solve these problems, it is conceivable to make the lowered position of the stacking plate not constant, but lower the stacking plate at the same timing irrespective of the amount of the stacked sheets and make the amount of lowering of the position of the uppermost surface of the sheets stacked on the stacking plate constant. There has been proposed a sheet feeding apparatus having a mechanism for moving up and down the stacking plate so as to make the lowering timing of the stacking plate the same and lowering the position of the uppermost surface of the sheets by a constant amount, as described above (see, for example, Japanese Patent Application Laid-open No. H02-152824 and U.S. Pat. No. 6,443,445).
However, the conventional sheet feeding apparatus having a construction in which the stacking plate is moved up and down so as to lower the upper surface of the sheets by a constant amount is very complicated in construction and requires many parts, and the cost as a sheet feeding apparatus increases remarkably. Further, the mounting of the many parts leads to the problem that the sheet feeding apparatus itself becomes bulky.